The invention relates generally to integrated circuits, and in particular to a cold cathode fluorescent lamp controller exhibiting a multi-function terminal.
Cold Cathode Fluorescent Lamps (CCFL) are extensively used today as backlights for computer liquid crystal displays (LCD) and LCD televisions. In certain embodiments CCFL lamps are driven in a push pull configuration such that current flows alternatively in each direction to ensure long life and uniform operation. A CCFL driver, and a controller operative with such a driver preferably provides a pair of complementary drive signals which are out of phase with each other. Typically, a CCFL driver requires a pair of driving signals which are 180° out of phase with each other, however this is not meant to be limiting in any way and 90° phase difference is also required in some particular applications.
Systems that use a plurality of CCFL lamps for a single display require synchronization between the CCFL drivers so as to avoid interference caused by non-synchronized operation. Further synchronization is often required with a system display signal, such as a video scanning signal, associated with a horizontal scanning frequency, so as to avoid a water wave effect. Such synchronization is required for single and multiple CCFL driver systems and thus requires an input terminal for receipt of any synchronization signal. Synchronization is typically accomplished with a phase locked loop which typically requires an external filtering capacitor accessed via another input terminal.
Ignition of a CCFL, also known as striking, requires a high voltage applied for a short period of time to ionize the gas within the fluorescent tube. Once ignition has been achieved a lower voltage is sufficient to maintain the CCFL in an on condition, known as the run state. CCFLs are often driven by a converter circuit, responsive to a CCFL controller, operative to convert an input DC signal to the AC waveform required to drive the CCFL in either the strike mode or the operating mode.
U.S. Pat. No. 6,979,959 issued Dec. 27, 2005 to Henry, the entire contents of which is incorporated herein by reference, is addressed to a lamp inverter with a continuous strike voltage exhibiting a frequency sweep generator. The frequency sweep generator sweeps the frequency of the lamp inverter to a striking frequency corresponding to a striking lamp voltage and then maintains the striking frequency until the lamp strikes. In certain embodiments, the frequency sweep generator is responsive to a voltage across an external capacitor, whose charge over time increases resulting in the sweeping of the frequency of the sweep generator. Such a capacitor is typically user selected, and of a value too large to be economically integrated within an integrated circuit. Thus, striking using a swept frequency requires an additional input terminal.
An integrated circuit providing the electronic control for striking and running the CCFL is termed hereinafter a CCFL controller.
A soft start function is also typically supported, which prevents start up over shoot during the strike mode. Such an over shoot is typically the result of an error amplifier indicating that substantially no current is flowing during strike mode until the lamp lights, and thus calling for increased on time for a pulse width modulation circuit. In the absence of a soft start function, upon ignition of the lamp a maximal amount of current will flow without limitation until the error amplifier senses the overflow and acts to reduce the current flow. Such a soft start function typically thus requires a timing capacitor, with yet additional associated input terminal.
In the event of an error condition being sensed by the CCFL controller, it would be advantageous to supply an error signal which may be sensed by the system electronics. Such an error signal requires yet another associated terminal.
In electronics the cost of each terminal of integrated circuit adds cost. Thus, it would be desirable to provided a multi-function terminal arranged to serve in place of at least some of the dedicated terminals of the prior art.